1. Field of the Invention
The invention generally relates to optical communication systems and in particular to an optical communication system capable of encoding information for transmission in a manner immune from eavesdropping.
2. Description of Related Art
A wide variety of optical transmission system and techniques have been employed to transmit information from one point to another. Often it is desirable to transmit the information in a manner which is immune from eavesdropping. Conventional cryptographic techniques have been employed to encrypt the transmitted information such that, if the optical transmission is intercepted, the information content of the message is difficult or impossible to decode.
A wide range of techniques and algorithms have been developed over the centuries for encoding data. However, it has been long been a goal of cryptography to transmit information which is substantially immune from eavesdropping yet does not require elaborate encryption techniques or algorithms. To this end, it has been proposed to transmit data within a noisy signal, wherein the noise obscures the encoded information to the extent that the information can not be extracted from the noise. By obscuring information within a noisy signal, the information is substantially immune from eavesdropping, even though the information may not be encoded subject to an encryption algorithm. Thus, the intended receiver of the information need only be capable of subtracting the noise from the signal to recover the information content. The receiver need not decode the message.
An exemplary optical transmission system employing such a technique modulates information onto an optical beam, then adds correlated noise to the beam. The beam is split into a pair of beams, by a conventional means such as a beam splitter. The correlated noise is sufficient to obscure the modulated information of the signal by producing a signal to noise ratio (SNR) of substantially less than one for either beam alone. The pair of beams are then transmitted independently to an intended receiver. The intended receiver combines the pair of signals to eliminate the correlated noise, thereby leaving only the correlated information content. To prevent the information content from also being eliminated, the information is preferably modulated onto the optical beams in complementary modes such that, when one optical beam is subtracted from the other, the information is not eliminated. To this end one of the optical beams may be "inverted" prior to transmission. Once the correlated noise is subtracted from the signals, the information signal remaining on the beams has a signal to noise ratio greater than 1, such that it can be demodulated from the beam. With this technique data is transmitted in a manner relatively immune of eavesdropping without requiring the use of an encryption algorithm. Hence, the intended receiver of the information need not employ any decoding technique to decipher the information, other than to subtract the correlated noise content from the pair of signals.
However, as a result of vacuum noise inherent in any optical transmission system, a fundamental minimum noise limit affects the resulting signal to noise ratio of the information signal. The vacuum noise limits the usefulness of the technique.